linux-kernel - Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set

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Message-ID: <20191009163835.GB19952@linux.intel.com>
Date:   Wed, 9 Oct 2019 09:38:35 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reto Buerki <reet@...elabs.ch>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for
 unrestricted guest

On Wed, Oct 09, 2019 at 12:40:53PM +0200, Paolo Bonzini wrote:
> On 27/09/19 23:45, Sean Christopherson wrote:
> > Rework vmx_set_rflags() to avoid the extra code need to handle emulation
> > of real mode and invalid state when unrestricted guest is disabled.  The
> > primary reason for doing so is to avoid the call to vmx_get_rflags(),
> > which will incur a VMREAD when RFLAGS is not already available.  When
> > running nested VMs, the majority of calls to vmx_set_rflags() will occur
> > without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS
> > during transitions between vmcs01 and vmcs02.
> > 
> > Note, vmx_get_rflags() guarantees RFLAGS is marked available.
> 
> Slightly nicer this way:
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 8de9853d7ab6..62ab19d65efd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1431,9 +1431,17 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>  void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> -	unsigned long old_rflags = vmx_get_rflags(vcpu);
> +	unsigned long old_rflags;
> +
> +	if (enable_unrestricted_guest) {
> +		__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
> +		vmx->rflags = rflags;
> +		vmcs_writel(GUEST_RFLAGS, rflags);
> +		return;
> +	}
> +
> +	old_rflags = vmx_get_rflags(vcpu);
>  
> -	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>  	vmx->rflags = rflags;
>  	if (vmx->rmode.vm86_active) {
>  		vmx->rmode.save_rflags = rflags;

Works for me.  Do you want me to spin a v3 to incorporate this and remove
the open coding of the RIP/RSP accessors?  Or are you planning on squashing
the changes as you apply?